1. Field of the Invention
The present invention relates to an array type oscillation device. More particularly, the present invention relates to an array type oscillator or oscillation device, that outputs electromagnetic waves containing at least frequency components in the frequency region from the millimeter wave band to the terahertz band (between 30 GHz and 30 THz) (to be referred to as terahertz waves hereinafter).
2. Description of the Related Art
Structures having a resonator and a negative differential resistance device are useful as devices for oscillating electromagnetic waves. For example, oscillation devices or oscillators that generate terahertz waves by means of a negative differential resistance device such as a resonant tunneling diode and an antenna are known. The oscillation output of such an oscillation device shows a tendency of decreasing its intensity as the frequency of the electromagnetic waves the device oscillates is raised toward the higher frequency side. For instance, while the output level of such a device is in the category of milliwatts for a frequency of about 100 GHz, the output level falls to get into the category of microwatts for a frequency of about 1 THz. Such a fall of output level is attributable to micronization of the junction area of the negative differential resistance device that is inevitably adopted in response to the use of a higher oscillation frequency.
Generally, in the field of negative differential resistance devices, the function of an RC low-pass filter is known to be realized by means of junction capacitance Cj and serial resistance Rs that corresponds to the resistance from the negative differential resistance device to the resonator. Namely, because higher frequency components of a signal are cut off by the function of an RC low-pass filter, structures of the above-described type cannot oscillate for electromagnetic waves in the frequency band higher than the cutoff frequency fc (=(2π×RsCj)−1). For this reason, the cutoff frequency that provides the upper limit of the frequency band of oscillation of an oscillation device has to be made higher than the frequency band of electromagnetic waves for which the device oscillates. Since the junction capacitance Cj is proportional to the junction area of the negative differential resistance device, the cutoff frequency fc can be raised by reducing the junction area. On the other hand, the electric power with which a bias power supply can feed the negative differential resistance device is proportional to the junction area of the negative differential resistance device. Therefore, as the oscillation frequency rises to a higher frequency, the power that can be fed to the negative differential resistance device falls to consequently reduce the oscillation output.
Formation of an array is known to be an effective means for achieving a sufficiently high output level for the purpose of broadening the scope of application of electromagnetic waves. Japanese Patent Application Laid-Open No. 2007-295350 discloses an exemplar configuration of oscillation device obtained by forming an array of a plurality of elements, which are so many sets of negative differential resistance devices and cavities. As shown in FIG. 14A of the accompanying drawings, a plurality of cavities 3 are formed on a substrate 2 and negative differential resistance devices 4 are arranged at the respective cavities 3. Note that the substrate 2 is laid on a resist layer 7 in FIG. 14A. As shown in FIG. 14B, bias lines 5 are employed to supply power from a bias power supply and electrically connected to the plurality of negative differential resistance devices 4. With such an oscillation device, as one of the negative differential resistance devices is driven to oscillate, adjacently located negative differential resistance devices are put in phase with the first negative differential resistance device and hence come to oscillate in phase with the first negative differential resistance device. Then, the electric power output from the elements 1 can be spatially synthesized (power synthesis) by utilizing this phenomenon. Thus, the above-described structure can provide a large oscillation output (power output) although the structure is relatively compact.
However, in the oscillation device disclosed in Japanese Patent Application Laid-Open No. 2007-295350, adjacently located negative differential resistance devices are directly linked to each other by bias lines. Therefore, isolation of the devices can hardly be secured in a low frequency region and hence the structure is accompanied by a difficulty of suppressing parasitic oscillations. Parasitic oscillations as used herein refer to oscillations induced in a lower frequency band that differs from the intended oscillation frequency band. Since such parasitic oscillations reduce the oscillation output at the intended frequency, suppression of parasitic oscillations that are intrinsically attributable to the structure of an oscillator, or oscillation device, using typical negative differential resistance devices provides an unavoidable problem that has to be solved.